The steering system for an engineering vehicle such as a wheel loader includes: a hydraulic pump; a steering cylinder driven by pressurized oil discharged from the hydraulic pump; and a steering valve for controlling the direction and flow rate of the pressurized oil supplied from the hydraulic pump to the steering cylinder. The steering system switches the steering valve based on the rotational direction and rotational quantity of the steering wheel so as to control the steering cylinder. In this case, the switching of the steering valve based on the rotational direction and rotational quantity of the steering wheel is performed by use of a hydraulic steering unit called “Orbit roll” (trade name). Here, the hydraulic steering unit includes: a hydraulic valve that operates in synchronization with the rotational operation of the steering wheel; and a hydraulic motor. The hydraulic steering unit is configured to generate a hydraulic pressure whose flow rate corresponds to the rotational quantity and rotational direction of the steering wheel.
In such a steering system for an engineering vehicle, typically, the hydraulic steering unit is located in a main circuit, and the steering valve is operated with the steering wheel to control the steering cylinder. In this case, a hydraulic valve of the hydraulic steering unit is provided as a steering valve, and the hydraulic pressure generated by the hydraulic valve unit is directly introduced into the steering cylinder.
In contrast to such a typical steering system for an engineering vehicle, a technology is known in which the hydraulic steering unit is used as an operation system of the steering valve so that the rotational operation of the steering wheel can be performed with a small force (for example, patent document 1, JP-U-1-154974). This kind of steering system uses a pilot pump as a hydraulic pressure source to introduce the oil pressure generated by a hydraulic steering unit into an oil-pressure switching unit (pressure receiving unit) of a steering valve so that the steering valve is switched. The steering valve is provided with a throttle line for converting the hydraulic pressure introduced from the hydraulic steering unit into the control pressure used for switching the steering valve.
On the other hand, there is also known a technology in which the operation system of the steering valve is formed by an electric/hydraulic steering unit including a controller and a solenoid valve so that the steering valve is electrically/hydraulically switched (for example, patent document 2, JP-A-10-45014). According to this technology, a rotational operation angle (steering angle) of the steering wheel is detected by a potentiometer, and the detected value is inputted into a controller. The controller outputs to the solenoid valve a command signal (electric signal) corresponding to the detection value. The solenoid valve, in turn, outputs a control pressure corresponding to the command signal. The control pressure is introduced into a hydraulic switching unit (pressure receiving unit) of the steering valve so that the steering valve is switched.
In addition, in the case of the steering system described in the patent document 2, a controller is provided with two or more kinds of predetermined gains (instruction signal/steering angle) of an instruction signal for a steering angle detection value. An operator is allowed to select one of the gains by operating a switch. This makes it possible to adjust the steering speed of wheels (the driving speed of a steering cylinder) with respect to the rotational operation of the steering wheel so that the steering speed matches both traveling and actual work. Therefore, the optimum steering operability can be achieved both during traveling and during actual work. In addition, there is also proposed a modified example in which a speed sensor for detecting the traveling speed is provided, and in which an instruction signal is consecutively changed in response to the traveling speed.
Patent document 1: JP-U-1-154974
Patent document 2: JP-A-10-45014